Our invention comprises improvements in known steering and braking systems for automotive vehicles wherein differential brake forces are applied to improve vehicle handling. In such systems, a turn can be achieved or altered by appropriately distributing brake pressure to the vehicle wheel brakes. The effective wheel braking forces at the left and right brakes determines direction of the turn and controls vehicle yaw, either simultaneously with driver controlled steering forces or independently of driver steering efforts.
Handling of the vehicle can be modified to produce a combined desired braking and cornering characteristic by varying the braking pressure at the wheels if either oversteering or understeering is detected. An understeering condition will result in an increase in the braking pressure of the front wheels. On the other hand, an oversteering condition will result in a decrease in the braking pressure of the rear wheels. These characteristics are described in U.S. Pat. No. 4,809,181.
The concept of controlling vehicle handling by varying wheel brake pressure has been extended to steering systems wherein the brake actuators are subjected to differential pressures in inside and outside wheel brake cylinders to develop a controlled yaw moment to promote turning maneuvers. This concept of using differential wheel brake pressures for steering purposes is described, for example, in U.S. Pat. No. 5,134,352.
U.S. Pat. No. 5,228,757 is a further example of a control system in which the steering behavior of the vehicle is controlled by differential wheel brake pressures, but it includes a sensor for detecting vehicle yaw rate as an operating variable for achieving a control signal that complements the driver steering command. The sensed vehicle yaw rate is compared to an estimated vehicle yaw rate based upon driver input. The controller determines the magnitude of any yaw rate error for purposes of developing a control signal that reduces the error. A decrease in yaw moment in the direction of the turn can be obtained by altering the left to right brake pressure distribution so that the brakes on the inside of the turn have less braking force than those on the outside. In effect, the controller establishes a correction to the desired yaw rate that is commanded by the driver steering wheel input. Corrections are made to the brake force distribution until a yaw rate reference value is achieved. Thus, during a combined hard braking in the steering maneuver, a braking yaw moment on the vehicle is imposed, thereby supplementing the control of the vehicle by the operator during hard braking.
In the prior art system of the '757 patent, a target yaw rate is computed based on steering wheel angle and vehicle speed. A yaw rate feedback is performed to modulate the distribution of braking pressure to the wheel cylinders.
The yaw moment is controlled directly in this prior art system by controlling the left to right pressure distribution, aside from the front and rear brake pressure distribution, during cornering.